As electronic goods, electronic devices, communication devices and the like have rapidly become smaller and lighter recently, and necessity of electric vehicles has highly emerged regarding environmental problems, demands for improving performance of secondary batteries used as a power source of these goods have greatly increased. Among these, lithium secondary batteries have received considerable attention as a high performance battery due to their high energy density and high standard electrode potential.
Particularly, lithium-sulfur (Li—S) batteries are a secondary battery using a sulfur series material having sulfur-sulfur (S—S) bonds as a positive electrode active material, and using lithium metal as a negative electrode active material. Sulfur, a main material of a positive electrode active material, has advantages of being very abundant in resources, having no toxicity and having a low atomic weight. In addition, a lithium-sulfur battery has theoretical discharge capacity of 1675 mAh/g-sulfur and theoretical energy density of 2,600 Wh/kg, which is very high compared to theoretical energy density of other battery systems (Ni-MH battery: 450 Wh/kg, Li—FeS battery: 480 Wh/kg, Li—MnO2 battery: 1,000 Wh/kg, Na—S battery: 800 Wh/kg) currently studied, and therefore, is a most promising battery among batteries that have been developed so far.
During a discharge reaction of a lithium-sulfur (Li—S) battery, an oxidation reaction of lithium occurs in a negative electrode (anode), and a reduction reaction of sulfur occurs in a positive electrode (cathode). Sulfur has a cyclic S8 structure before discharge, and electric energy is stored and produced using an oxidation-reduction reaction in which an oxidation number of S decreases as S—S bonds are broken during a reduction reaction (discharge), and an oxidation number of S increases as S—S bonds are formed again during an oxidation reaction (charge). During such a reaction, the sulfur is converted to linear-structured lithium polysulfide (Li2Sx, x=8, 6, 4 and 2) from cyclic S8 by the reduction reaction, and as a result, lithium sulfide (Li2S) is lastly produced when such lithium polysulfide is completely reduced. By the process of being reduced to each lithium polysulfide, a discharge behavior of a lithium-sulfur (Li—S) battery shows gradual discharging voltages unlike lithium ion batteries.
Among lithium polysulfide such as Li2S8, Li2S6, Li2S4 and Li2S2, lithium polysulfide having a high sulfur oxidation number (Li2Sx, commonly x>4) is particularly readily dissolved in a hydrophilic liquid electrolyte. Lithium polysulfide dissolved in the liquid electrolyte is diffused away from a lithium polysulfide-produced positive electrode due to a concentration difference. Lithium polysulfide eluted from the positive electrode as above is washed away out of the positive electrode reaction area making it impossible to be gradually reduced to lithium sulfide (Li2S). In other words, lithium polysulfide present in a dissolved state outside the positive electrode and the negative electrode is not able to participate in charge and discharge reactions of a battery, and therefore, the sulfur material amount participating in an electrochemical reaction in the positive electrode decreases, and as a result, it becomes a main factor causing a charge capacity decrease and an energy decrease of a lithium-sulfur battery.
Furthermore, apart from those floating or immersed in the liquid electrolyte, lithium polysulfide diffusing to the negative electrode directly reacts with lithium and is fixed on the negative electrode surface in a Li2S form, which causes a problem of corroding the lithium metal negative electrode.
In order to minimize such lithium polysulfide elution, studies on changing morphology of a positive electrode composite filling various carbon structures with sulfur particles have been ongoing, however, such methods are complicated in the preparation and have not resolved fundamental problems.